There are a number of conditions in the heart which necessitate monitoring the cardiac tissue for sources of abnormal electrical activity within the heart and/or which require ablation of tissue within the heart where such sources of electrical activity are located.
Two such conditions are atrial fibrillation and ventricular tachycardia. Atrial fibrillation is a condition in the heart in which abnormal electrical signals are generated in the endocardial tissue to cause irregular beating of the heart. One method used to treat atrial fibrillation involves creating several long (i.e. approximately 2-10 cm) lesions on the endocardium within the atria. These lesions are intended to stop the irregular beating of the heart by creating barriers between regions of the atria. These barriers halt the passage through the heart of the abnormal currents generated by the endocardium. This procedure is commonly referred to as the "maze procedure" because it creates a maze of lesions design to block the passage of abnormal currents through the heart.
Existing procedures for forming such linear lesions include the highly invasive technique of opening the patient's chest and heart and forming linear incisions inside the atria. Naturally, the highly invasive nature of this procedure makes it a particularly high risk to the patient and necessitates extraordinarily long recovery time.
Other attempts have been made to form the linear lesions using ablation catheters fed into the heart via the patient's vessels (i.e., the arteries or veins). For example, one such procedure involves inserting into the atria a 7 French catheter having an ablation tip. Radio frequency (RF) energy is supplied to the tip as the tip is dragged across the endocardium, thereby burning linear lesions into the endocardium.
While often successful for forming linear lesions, the ablation tip of the catheter can sometimes lift off of the surface of the endocardium as it is dragged across the endocardium, creating one or more breaks in the lesion. Such breaks minimize the success of the ablation procedure by leaving a path through which current may travel during atrial fibrillation episodes.
Ventricular tachycardia is another condition which generates abnormal electrical activity in the heart and which can require ablation of cardiac tissue associated with the abnormal electrical activity. Ablation of tissue for ventricular tachycardia may be performed using RF energy delivered by an electrode positioned at the tip of an ablation catheter. Typically, the lesions formed by the ablation tip must extend deeply into the tissue and so good contact between the tip electrode and the tissue is important.
In patients experiencing atrial fibrillation and ventricular tachycardia, it is often desirable to map the electrical activity of the cardiac tissue in order to determine the location of the irregular electrical activity so that ablation procedures may be carried out at the appropriate location. One type of mapping catheter utilizes an expandable basket, plaque, helix, coil, or other structure positioned at the distal end of a catheter and a plurality of electrodes carried by the expandable structure.
The expandable structure is initially in a collapsed condition and is fed via the patient's vessels into the chamber of the heart which is to be mapped. Once inside the chamber, the expandable structure is released or moved into its expanded condition and it is positioned such that the electrodes are in contact with the cardiac tissue within the chamber. The electrical activity at each electrode site is monitored and maps showing the electrical activity at various points within the chamber may be produced.
As with ablation procedures, better results are achieved during endocardial mapping procedures if the mapping electrodes are securely supported against the endocardial tissue. If insufficient contact is made between the electrodes and the tissue, the electrical activity of the tissue beneath those electrodes will not be properly recorded.
Procedures and devices for ablating and/or mapping endocardial tissue are therefore desired which utilize catheters having sufficient flexibility and maneuverability to allow introduction of the electrodes into the cardiac chamber with minimal tissue trauma, but which hold the mapping and/or ablation electrodes securely against the target tissue which is to be mapped and/or ablated.